1. Field of the Invention
This invention pertains to non-destructive measurement techniques of microroughness cross-correlation properties between boundary layers in multilayered dielectrics. In particular, this invention pertains to a method for evaluating the overall cross-correlation nature of multi-layered dielectric stacks based on comparisons between the measured angular distribution of scattered light and the calculated distributions of scattered light for various dielectric stack models. The various models are based on different cross-correlation properties.
2. Description of Prior Art
In high quality optical systems, such as mirrors handling large amounts of energy, it is important to keep mirror absorption as low as possible to prevent deformation of the mirror surface. To decrease absorption, modern optical systems use multilayer dielectric components which permit higher reflectance levels than for uncoated components. Even where high energy beams are not required, scattered light can seriously degrade the performance of modern optical systems. Examples include the ring laser gyro where retroscattered light is catastrophic.
Previous methods of establishing cross-correlation properties of dielectric stacks included profilometer measurements of each layer after deposition. This is time consuming, very inconvenient and not practical on production dielectric stacks. Another method involves destruction of the stack by cleavage and making visual inspection. This is also not practical since it destroys the stack it measures. Since dielectric stacks are used in optical systems, it is preferred to evaluate the cross-correlation properties in an optical manner.
The angular distribution of scattered light has been evaluated on a theoretical basis for uncoated surfaces and surfaces with a single dielectric layer. In the case of multilayer dielectric stacks, a scalar theory calculation of total integrated scatter, TIS, has been made. The TIS calculation does not retain the vector nature of the electromagnetic fields whereas a vector theory does. The TIS theory relates the root mean square, rms, microroughness to the total amount of scattered light and does not involve measurement of the angular distribution of scattered light. When calculating the angular dependence of scattering from multilayer dielectrics, the autocorrelation and cross-correlation functions associated with the microroughness of the dielectric stack interfaces are needed. These functions provide much more information about the surface than does the rms microroughness value alone. The TIS calculation is based on the scalar Kirchhoff diffraction integral. The relationship between the rms microroughness and TIS is given by ##EQU1## where R.sub.o is a fraction of the incident light which is reflected into all angles including a specular direction, R is the fraction which is specularly reflected, .delta. is the rms height of the surface microirregularities, and .lambda. is the wavelength. This relation has been shown to correctly predict the TIS from real optical surfaces including polished and diamond turned metal surfaces.
FIG. 1 shows a typical TIS configuration where a light source 10 emits light on an optical surface 12 which undergoes scattering 14. A collector 16 reflects scatter 14 to a detector 18 which gives a total value of the scattered light. Prior publications describing the above techniques can be found in "Scalar Scattering Theory for Multilayer Optical Coating" by C. K. Carniglia, "Optical Engineering", Vol. 18, No. 2, March/April 1979, page 104, and "Surface Scattering in Optical Interference Coatings", J. M. Eastman, 1974, University Microfilms, Int., Doctoral Thesis Univ. of Rochester.
The TIS measurement has no known method of evaluating the interface cross-correlation properties of multilayer stacks. The previous calculation involving a single dielectric layer could be used to evaluate the cross-correlation properties. This calculation is a special case of the general multilayer theory which is the basis of this invention. The angular distribution of scatter provides an evaluation tool as to whether the multilayer stack has predominantly correlated, partially correlated or uncorrelated interface microroughness. In general, uncorrelated layers provide lower overall scatter, at the design wavelength, from a mirror in a device such as the ring laser gyro than does a correlated dielectric stack. This is because correlated layers produce constructive interference in certain regions of the scattering hemisphere whereas uncorrelated layers tend to yield scattered light with random phase behavior.